High intensity discharge (HID) lamps as are used in commercial and industrial installations because of their high luminous output as well as their long life characteristics utilize an arc tube having a fill contained therein excited to a discharge state upon introduction of the appropriate energy being supplied thereto. The fill contained within the arc tube is under pressure, typically on the order of about 10 atmospheres during operation (or 7600 torr). For safety purposes as well as for thermal management properties as is generally known in the lighting field, the arc tube will be surrounded by a shroud typically made of quartz and then such arc tube and shroud configuration is mounted within an outer lamp envelope which is evacuated and has a base portion attached thereto for supplying the appropriate energy. For an example of such a discharge lamp configuration having a shroud surrounding the arc tube, reference is made to U.S. Pat. No. 5,122,760 issued to Parrot et al. on Jun. 16, 1992.
With respect to the safety function of serving as a containment device in the event of a rupture of the arc tube, the shroud device has been proven to be effective to the extent of allowing the use of shrouded metal halide lamps in open fixtures, that is, fixtures that do not require an expensive cover plate. In utilizing the shrouded configuration, however, further consideration must be given to the problem of sodium loss from the fill as may occur as a result of a negative charge which is present on the arc tube walls and is caused by photoelectric emission from the electrified side rods used to support the arc tube and shroud within the outer lamp envelope. Various arrangements for solving this problem have been proposed and can be found in U.S. Pat. Nos. 4,963,790, 5,122,706, 5,136,204, 5,252,885, and, 5,270,608. Although the lamp mounting arrangements of these patents provide improvement over the then existing solutions to the support and sodium loss problem, such configurations suffer in terms of size due to the fact that all such arrangements utilize support members disposed outside of the shroud thereby increasing the need for space within the outer envelope. One solution to this size problem has been advanced by the teachings of U.S. patent application Ser. No. (Assignee Docket No. LD 10621) entitled "Lamp Assembly with Shroud Employing Insulator Support Stops" which is assigned to the same assignee as the present invention and is herein incorporated by reference.
It has been found that with the previous approaches to the problem of supporting the shroud and arc tube within the outer envelope, including the solution proposed in the above-referenced U.S. patent application Ser. No. (Assignee Docket No. LD 10621), there is a need for highly precise machining operations to be performed on the support members as well as the shroud so as to insure a sufficient fit between the various components that make up the arc tube, shroud and supporting component configuration. In fact, because of the tight fit necessary between the support members and the shroud itself, it has been found that a fit which is too tight will result in breakage of components during manufacture whereas a fit which is not tight enough can result in vibration of the component parts during operation of the lamp. In many cases, the cause of this problem is due to the inelastic nature of the respective component parts. In such an instance, the arc tube itself would serve as the tension member whereas the insulator support members would serve as compression members. The use of inelastic materials in both the tension members and the compression members results in a loose contact between all of the components. Such an inelastic structure is not robust to shock and may result in failure and further places stress on the arc tube seal structures. Therefore, it would be advantageous if a lamp assembly having a shroud disposed in surrounding relation to the arc tube could be provided that would have a support structure wherein components other than the arc tube would provide the resiliency needed to withstand the operating conditions of the lamp. It would be further advantageous to allow for a manufacturing process that was relaxed in terms of the precision of the machining requirements of the various components yet still achieving a product of the highest quality at an economical manufacturing cost of production.